In order to assess the role of the thymus in the acquisition of the T cell self-MHC specific repertoire, the MHC specificity of cytotoxic precursor T cells differentiating with the thymii and spleens of chimeric and thymus-engrafted nude mice was determined. The results demonstrated that cytotoxic T lymphocytes resident within the thymii of these experimental animals recognize TNP only in association with thymic MHC determinants. In contrast, cytotoxic T lymphocytes from the spleens of these same mice recognize TNP on both thymic and non-thymic host type MHC determinants. These results are consistent with the thymus being one site which determines the specificity of the T cell repertoire, but suggests that peripheral sites also play a critical role. Specifically, T cells specific for self-I are solely educated intra-thymically whereas T cells specific for self-K/D are educated both intra-thymically and extra-thymically.